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"(00:00) in this video we will be discussing an important factor in reservoir dynamics known as the skin effect here is a quick overview of what we will be explaining what is the skin effect negative versus positive skin factor damaged formation stimulated formation open hole versus case completion wells total skin factor skin factor's effect on flow rate and why it matters with all that said let's begin with what is the skin effect the skin effect is defined as a dimensionless factor that attempts to resolve the difference between ideal (00:37) theoretical well performance and the reality skin is a very convenient way of evaluating and ranking oil performance which is the ability of fluid to flow into or out of the well this well performance is influenced by the resistance of the formation the viscosity of the fluid and the additional resistance concentrated around the wellbore resulting from the drilling and completion technique employed the damage done to the weld during drilling operations creates a lower permeability in the immediate area surrounding the wellbore than the (01:07) formation this results in a discontinuity in the flow of the fluids once this barrier is reached and the greatest pressure drop takes place in the fluids flowing into the well now that we have described the skin effect let's move on to the two types of skin and their properties positive skin indicates a decrease in permeability in the area surrounding a wellbore this area of the formation is considered to be damaged and results in an increased pressure drop and a decrease in flow rate negative skin value indicates an (01:39) increase in permeability in the area surrounding the wellbore hence a stimulated reservoir positive skin factors result in a decreased pressure drop across the skin region and an increase in flow rate it is impractical to have large negative skins because it results in an infinite flow rate we know that ln of the effective radius over the wellbore radius is about eight therefore a skin factor of negative eight would give an infinite flow rate as mentioned in the previous slide there is a negative skin factor indicating (02:08) stimulation and a positive factor indicating damage the grass picture depicts the pressure drop across the reservoir as fluid flows towards the wellbore the graph on the left shows a damage formation it can be seen as the flow reaches the skin region pressure significantly drops thus there is an increase in the total pressure drop the graph on the right shows a stimulated formation it can be seen as the flow reaches the skin region pressure increases thus the total pressure drop is decreased so now let's talk about damage and (02:41) stimulation in a little more detail let us begin with damage formations reduction of permeability can be caused by the invasion of drilling fluids into the section of the reservoir surrounding the wellbore the mud cake and mud filtrate act together to reduce permeability the drilling fluid invades the pore space partially plugging the invaded zone additionally when water-based filtrates from drilling completion work over or stimulation fluids into the formation clays may swell which clog additional pore space reducing permeability (03:14) along with this ion exchanges and changes in salinity may also cause swelling before casing is run in an open hole the mud filtrate invades a pore space near the wellbore and is lost to the formation during the cementation process the cement slurry usually has a significantly higher density than the mud therefore there is a tendency for invasion to occur after cementing the well perforating is required to reduce the well during this process the perforating itself also does some damage to the formation due to the crushing of the (03:48) rocks surrounding the perforations when they are fired now we will review a quick video from clearing oil services to help visualize the effects skin has on flow rate and production levels ideally during production of oil and gas wells there is no skin factor this is when production levels are at their maximum as the pores near the wellbore continue to clog and the permeability decreases the skin factor becomes more positive as shown this results in lower flow rates and ultimately decreased production levels contrary to a damaged formation it can (04:30) also be stimulated which increases permeability by two methods acidizing and fracturing stimulating the well results in a negative skin factor increasing the effective world war radius and therefore increasing production acidizing is the process of injecting acid into the formation and allowing it to dissolve the material clogging the pore space matrix acidizing is typically used in repairing skin damage because it only affects the area a few feet from the wellbore it is the process of injecting acid at rates below the fracture pressure (05:04) hydraulic fracturing on the other hand is another method used in stimulating the well large volumes of fracture fluid are injected into the well to fracture the reservoir rock creating highly permeable flow paths to the world war although past studies have shown a negative skin on hydraulically fractured wells the skin directly surrounding the wellbore may actually be significantly damaged from the fracturing itself however the well has changed so drastically that the direct paths of permeability ultimately still produce a (05:34) negative skin factor this is a demonstration of hydraulic fracturing stimulating a welt as the fracture fluid is injected into the reservoir it creates fractures that serve as paths of high permeability to the world war another factor influencing the skin factor is the difference between an open hole and a cased well ideally every well would be an open hole completion well they are less expensive because they do not require the well to be cast and cemented all the way to the bottom of the well which cost extra (06:07) money open hole completion wells also have much lower skin factor because there's not cement invading the formation despite this open hole completion is not always an option due to the possibility of formation fluid entry into the wellbore many wells must be cased and cemented similarly to the drilling fluid invasion semen is then likely to invade the formation and plug pore space this creates a more positive skin factor all of these factors discussed result in additional pressure drop which is called completion pseudo skin factor (06:41) all of the factors we have discussed affect the total skin factor this total skin factor accounts for the difference between the well performance of an actual well and that of an ideal vertical open whole wealth total skin factor equals the skin factor due to formation damage or stimulation plus the completion pseudo skin due to partial penetration plus completion suit of skin due to perforations plus the skin due to the rock compaction around perforation tunnels plus geometrical pseudo skin due to inclination plus the completion suit of (07:16) skin due to hydraulic fracturing this is necessary to calculate the total pressure drop across the skin region skin factor ultimately becomes into play in reservoir dynamics when applied to the flow rate equation this chart shows that fluid produced first time is related to the different values for the skin effect although it may be counter-intuitive that the negative skin factor would create a higher flow rate it can be seen that if you plug in a negative skin factor the denominator becomes smaller which will increase the rate of flow (07:47) the smaller value of skin factor the greater the rate of flow is as the skin factor becomes larger more positive the flow rate is decreased as stated previously skin factor plays a large role in determining flow rates the chart shown displays how the damage or stimulation of the area near the wellbore greatly affects the well's production rates when the skin region is damaged or has a positive value a well has a lower production rate due to a decrease in permeability and an additional pressure drop in dealing with the negative skin the (08:21) effective wellbore radius is increased beyond the initial radius of the weld this shows how fracturing or acidizing the formation increases permeability and decreases pressure drop which in turn increases flow rate this is a demonstration of how skin affects the flow rate in a petroleum reservoir the marbles represent the hydrocarbons and the top of the box represents a portion of the reservoir with a high path of permeability and no skin factor the lower portion represents the part of the reservoir with damage that has a (08:52) lower permeability and a positive skin factor the flow of hydrocarbons through the undamaged portion of the reservoir is significantly higher than the lower portion where the hydrocarbons are deflected by clogged pores this is represented here by pieces of cardboard here is a brief video of our demonstration that has been slowed down to help show the effect skin has on flow rate in conclusion the skin factor has a direct impact on flow rate and ultimately production positive skin indicates a damaged well which hinders flow rate while a negative (09:46) skin increases flow rate if skin factors are not properly accounted for the necessary corrections cannot be performed on the reservoir ultimately hindering production time if the well is not flowing at maximum efficiency longer time will be required to produce the hydrocarbons which consequently will cost the company more money you"